How LUTs Work: 1D vs 3D Lookup Tables, Cube Files and Log Footage

A lookup table, or LUT, is a fixed list of color conversions. It takes each input color and returns a specific output color. Editors use LUTs to convert footage from one color space to another, to apply a consistent look across many clips and to speed up grading. A LUT does not analyze the image. It applies the same mapping to every pixel, so its results depend on how well the input matches what the LUT expects.

The basic idea

Imagine a table with one entry for every possible color. The input color is looked up, and the matching output color replaces it. Because the table is fixed, applying it is fast and predictable. The same LUT on the same input always gives the same output.

Real images contain millions of possible colors, so LUTs do not store every entry as a separate line. They store a sample grid and interpolate between the samples. The size of that grid determines how precise the mapping can be and how large the file becomes.

1D and 3D LUTs

Editors meet two main types of LUT. A 1D LUT maps each color channel separately. Red, green and blue each have their own curve, so a 1D LUT can adjust tone and contrast but cannot shift the relationship between colors. A 3D LUT maps the combined red, green and blue values together, which allows changes that depend on the color of the pixel.

Because a 3D LUT can change hue and saturation based on the pixel's color, it is far more flexible than a 1D LUT. It is also the usual choice for converting between camera log formats and display color spaces, and for creative looks that shift colors in a specific way.

Common 3D LUT sizes

A 3D LUT is defined by its size per axis, usually written as a cube. A size of 17 means the table has 17 steps for red, 17 for green and 17 for blue. The total number of entries is the size cubed. The table below shows the entry counts for three common sizes.

Grid size	Total entries	Typical use
17 x 17 x 17	4,913	Quick previews, light files
33 x 33 x 33	35,937	Common balance of accuracy and size
65 x 65 x 65	274,625	Fine detail, large or precise conversions

Larger grids give more accuracy in areas where the color changes quickly. They also cost more memory and processing time. For most creative work, a 33-point grid is a sensible default, and a larger grid is worth using only when precise conversion matters.

The cube file format

The .cube file is a common plain-text format for 3D LUTs. It lists the size of the grid and then gives the output color for each entry. Because it is plain text, it can be opened and checked in a text editor, although its values are long and difficult to read by hand.

Many grading and editing programs can load .cube files. Check that the program expects the same input and output color space as the LUT was designed for. A LUT made for one camera log format may look wrong if applied to a different format, even though the file itself loads without error.

Log footage and why LUTs are used

Many cameras record footage in a log format. Log encodes a wide range of brightness into the file, which preserves detail in the highlights and shadows. The result looks flat and gray on a standard display, because the image has not yet been converted into a standard viewing space.

A conversion LUT maps log footage into a standard space such as Rec. 709 for high-definition display. Once the conversion is applied, the image appears with normal contrast and color, and the editor can grade it from a familiar starting point. Many camera manufacturers publish conversion LUTs for their log formats, which makes the first step of grading faster and more consistent.

Creative LUTs and technical LUTs

Technical LUTs perform a conversion. They are meant to move footage from one defined space to another, and they should not change the look of the scene beyond that conversion. Creative LUTs add a stylized look on top of the conversion. They may shift colors, add contrast or create a film-like feel.

Keep the two kinds separate in your workflow. Apply the technical conversion first, then add a creative LUT or a manual grade. If the creative look is applied directly to log footage, the result can look harsh or unnatural because the LUT expects a different input.

Using LUTs without losing control

A LUT is a starting point, not a finished grade. After applying a LUT, check the skin tones, the highlights and the shadows with scopes. Many LUTs push saturation or contrast too far for a particular shot. Reduce the strength of the LUT if it pushes colors too hard, or add primary corrections after the LUT to balance the result.

Compare a LUT on several shots from the same scene. A LUT that works on a bright exterior may look wrong in a dim interior. Matching the shots after applying the LUT keeps the scene consistent.

Checking a LUT before trusting it

A LUT can be wrong in ways that are easy to miss on a single frame. Start with a neutral test chart or a shot that includes a gray card, if one was recorded. A gray card should remain neutral after the conversion, and a clear color cast after the LUT means the input or output space does not match what the LUT expects.

Next, look at skin tones on the vectorscope. Skin should fall near the usual skin tone line, not drift toward green or magenta. Finally, check the highlights on the waveform. A LUT that clips the top of the signal removes detail that cannot be recovered later. Catching these problems early keeps the grade honest and avoids repeated fixes downstream.

Storage and sharing

LUT files are small compared with video, so they are easy to store and share. Keep a folder for technical LUTs and a separate folder for creative looks, and name each file with the camera, the log format and the intended output. A clear name prevents errors when a project is handed to another editor.

Record which LUTs were used on each project. A project note that lists the input format, the conversion LUT and any creative LUT makes future revisions faster and protects the look if the project is reopened months later.

Key takeaways

A LUT is a fixed mapping from input colors to output colors. A 1D LUT adjusts each channel separately, while a 3D LUT maps the combined colors and is far more flexible.

Use a technical LUT to convert log footage, then add creative looks or manual grades on top. Choose grid sizes by balancing accuracy against file size, and check the result with scopes.

Treat every LUT as a starting point. Test it on several shots, adjust its strength when needed and keep notes so the look can be repeated later.